Control arrangement and method for power electronic system

ABSTRACT

A control arrangement and method is provided that monitors the condition and operating parameters of a power electronic system having power electronic devices and responds to various detected abnormalities via appropriate action to optimize operation of the power electronic system. The arrangement increases reliability of operation and optimizes the continuous supply of power to a load. For example, the arrangement responds to an overheated power electronic switch by switching to an alternate power electronic switch, the overheated power electronic switch being made available as a temporary alternate path. The arrangement also includes the capability for diagnosing the power electronic switches by measuring the temperature of the power electronics switches and communicating the temperature information in a coordinated fashion with information regarding the operating status of the power electronics switches. Additionally, shorted power electronic switches are detected and appropriate action taken.

This application claims the benefit of U.S. Provisional Application No.60/131,724 filed on Apr. 30, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of power electronicsystems and more particularly to control methods and arrangements thatmonitor the condition and operating parameters of the power electronicsystem and power electronic switches and provide appropriate action tooptimize operation thereof.

2. Description of Related Art

Various power electronic systems are known for supplying power,regulating power, and transferring power from one source to another inorder to provide continuous power to a load. Ascertaining the properoperation of the various components of these systems is important inorder to most appropriately decide how to best assure the continuoussupply of power to the load. While these arrangements may be useful andgenerally satisfactory for their intended purposes, they do not provideappropriate diagnostics or system control with sufficient emphasis onthe priority of the continuous supply of the connected load.

SUMMARY OF THE INVENTION

Accordingly it is a principal object of the present invention to providea control method and arrangement that monitors the condition andoperating parameters of a power electronic system having powerelectronic devices and responds to various detected abnormalities viaappropriate action to optimize operation of the power electronic system.

It is another object of the present invention to provide a controlmethod and arrangement for a source transfer control system thatresponds to an overheated power electronic switch by switching to analternate source, the source with the overheated power electronic switchbeing made available as a temporary alternate source.

It is another object of the present invention to provide a controlmethod and arrangement for a source transfer control system havingmonitoring and control features to increase reliability of operation andto optimize the continuous supply of power to a load.

It is yet another object of the present invention to provide adiagnostic arrangement for a power electronics system including powerelectronic switches that measures the temperature of the powerelectronics switches and communicates the temperature information in acoordinated fashion with information regarding the operating status ofthe power electronics switches.

These and other objects of the present invention are efficientlyachieved by a control method and arrangement that monitors the conditionand operating parameters of a power electronic system having powerelectronic devices and responds to various detected abnormalities viaappropriate action to optimize operation of the power electronic system.The arrangement increases reliability of operation and optimizes thecontinuous supply of power to a load For example, the arrangementresponds to an overheated power electronic switch by switching to analternate power electronic switch, the overheated power electronicswitch being made available as a temporary alternate path. Thearrangement also includes the capability for diagnosing the powerelectronic switches by measuring the temperature of the powerelectronics switches and communicating the temperature information in acoordinated fashion with information regarding the operating status ofthe power electronics switches. Additionally, shorted power electronicswitches are detected and appropriate action taken.

BRIEF DESCRIPTION OF THE DRAWING

The invention, both as to its organization and method of operation,together with further objects and advantages thereof, will best beunderstood by reference to the specification taken in conjunction withthe accompanying drawing in which:

FIG. 1 is a block diagram representation of a power electronic systemutilizing the control arrangement of the present invention;

FIGS. 2-8 are diagrammatic representations of signals at various pointsin the system of FIG. 1;

FIG. 9 is a one-line, block diagram representation of a powerelectronics switching system utilizing the control arrangement of thepresent invention; and

FIG. 10 is a one-line, diagrammatic representation of portions of asolid-state switch of FIG. 9.

DETAILED DESCRIPTION

Referring now to FIG. 1, the control arrangement and method of thepresent invention will be described in connection with an illustrativesystem 15 that includes a controller 18 that monitors the condition andoperating parameters of various components of the system 15 and takesappropriate action to optimize operation thereof, e.g. the operatingcharacteristics of an illustrative electronic switch stage 10 aremonitored as will be explained in more detail hereafter. As illustrated,the electronic switch stage 10 includes a main path between lines 12 and14 that is controlled between on and off states, corresponding torespective conductive and non-conductive states, via a controlconnection at 16. In a specific illustrative example, the electronicswitch stage 10 is a thyristor, IGBT, TRIAC, pair of inverse-parallelconnected SCR's, or other actively controlled device.

The system 15 includes an illustrative communications arrangement 22that cooperates with the controller 18 to provide information to thecontroller 18 over communications lines at 20, which in specificembodiments is formed by one or more data buses and/or control lines. Inthe illustrative embodiment, the communications arrangement 22 includesa switch control/monitor stage 30 that is located in the vicinity of thesystem component to be monitored, e.g. the electronic switch stage 10,and that transmits monitored information to a communicationsencoder/multiplexer stage 26, “comm. encoder/mux” 26 hereafter, via acommunications link 28, e.g. a dielectric medium such as fiber optics ina specific embodiment. As illustrated, where multiple components aremonitored by the system 15, multiple switch control/monitor stages 30are provided along with multiple communication links 28, e.g. 28 a, 28b. The comm. encoder/mux stage 26 then functions to multiplex theinformation on the various communication links 28 and provides theinformation in a predetermined multiplexed format at 20 to thecontroller 18.

The control connection 16 of the electronic switch stage 10 is connectedto a gate drive signal at 24 provided by the switch control/monitorstage 30. In this illustrative example, the system 15 monitors the gatedrive signal at 24 and/or the temperature of the switch stage 10 viadata at 32. This arrangement is especially useful where the illustrativeelectronic switch stage 10 or various other component is locatedremotely from the controller 18 and/or where the illustrative electronicswitch stage 10 is located in a more severe environment that isdeleterious for the controller 18, e.g. high-noise, medium voltage,high-temperature etc. In one specific embodiment, the temperature of theswitch stage 10 is measured at the location of the switchcontrol/monitor stage 30 with the switch control/monitor stage 30 beingin the proximate vicinity of the switch stage 10, e.g. on a commonmounting arrangement or heat sink 34 (not shown in detail).

Considering now an illustrative embodiment of the communicationsarrangement 22 of the system 15 and referring now additionally to FIG.2, the information on the communication link 28 includes arepresentation of the gate drive signal 24, such that a pulse signal 40is sent over the communications link 28 when the electronic switch stage10 is conducting. The pulse signal 40 is sent on a repetitive basis,e.g. each basic clock cycle or each half-cycle of a fundamental waveformthat is present on the line 12 to the electronic switch stage 10. Thereceipt of this signal 40 by the comm. encoder/mux stage 26 and thetransmission of this representation to the controller 18 over lines 20also indicates that the communications arrangement 22 is operational andthat the electronic switch stage 10 is not shorted.

In the illustrative embodiment of FIG. 1, the electronic switch stage 10is one stage of an overall series-connected electronic switch, e.g. sixstages as depicted in FIG. 1 by a second stage 10 a and a sixth stage 10b. Also provided for each stage is one of the switch control/monitorstages 30, e.g. 30, 30 a, 30 b which transmits a signal on each of thecommunication links 28, e.g. 28, 28 a and 28 b, to the comm. encoder/muxstage 26. For example, as depicted in FIG. 2, respective signals 42 and44 are transmitted for the second and sixth electronic switch stages 10a and 10 b which are generated simultaneously and repetitively. Thecomm. encoder/mux stage 26 then multiplexes the received pulse signals,e.g. 40, 42 and 44, and provides the multiplexed signal at lines 20 tothe controller 18. Accordingly, the receipt by the controller 18 of thecontinuous train of pulses verifies that each switch stage of the stages10, 10 a, 10 b etc., denoted as 10 x hereafter, is conducting. If thepulses are not continuous, e.g. not present in the predetermined patternand spacing as shown in FIG. 3, i.e. one or more of the pulses aremissing at the periodic rate, then the controller 18 is advised/alertedthat something is wrong with either one of the electronic switch stages10 x or the communication arrangement 22. If the pulse train ofmultiplexed signals at 20 is synchronized to the controller 18, thecontroller 18 can identify which of the stages has a malfunction, e.g.stage 3 in FIG. 3 as indicated by the missing pulse denoted 62.

Considering now an illustrative embodiment where additional informationis transmitted over the communications arrangement 22 and referring nowadditionally to FIG. 4, it is desirable for the controller 18 toascertain additional information about the various components of thesystem 15, e.g. the temperature of the electronic switch via the sensedtemperature signal 32. To accomplish the communication of additionalinformation, the switch control/monitor stage 30 encodes additionalinformation along with the gate driver signal information, e.g. as shownin FIG. 4 by the addition of a pulse signal 50 that representstemperature of the electronic switch stage 10 along with arepresentation of the gate driver signal, e.g. pulse signal 52. In aspecific arrangement, the width of the pulse 50 is proportional to thesensed temperature at 32. Thus, the pulse signals 50, 52 are sent overthe communications link 28 on a periodic basis, e.g. as discussedbefore, for each basic operational cycle of the system 15. For example,pulse signals 50, 52 correspond to a switch control/monitor stage 30associated with a first electronic switch stage 10 and pulse signals 54,56 correspond to the stage 30 a associated with a second electronicswitch stage 10 a. It should be noted that in FIG. 4, while the pulsesare shown sequentially for each stage, the pulses for each of the stagesis sent repetitively and simultaneously, the representation in FIG. 4being the multiplexed sequential arrangement performed by the comm.encoder/mux stage 26 in response to the continuous information receivedfrom the various stages on the communication links 28, 28 a, 28 b etc.

In a specific embodiment, the comm. encoder/mux stage 26 alsoincorporates an ambient temperature signal to the controller 18. Forexample, with additional reference to FIG. 5, after the comm.encoder/mux stage 26 outputs a sequence of pulses corresponding to eachof the stages, an ambient temperature signal 60 is encoded ormultiplexed into the pulse train in place of the first stage signal orother position. Thus, the controller 18 receives a pulse train ofsignals representing the gate signal and the temperature of each of theswitch stages 10 x followed by the ambient temperature of theenvironment of the controller 18 and the comm. encoder/mux stage 26. Inthis manner, the temperature rise of each switch stage 10 above theambient temperature is available. Additionally, as shown in FIG. 5, theabsence of a pulse signal for any of the stages, e.g. at 63 for stage 3,indicates a malfunction of the communications link or the gate drivesignals or the shorted condition of the respective switch stage 10 etc.

In accordance with additional aspects of the present invention, andreferring now additionally to FIG. 6, in a preferred embodiment, thegate driver signal pulse 40 is transmitted over the communications link28, on a normal basis in one specific embodiment, or in another specificembodiment, upon a requested basis as determined by the controller 18.For example, the controller 18 issues a request signal, as illustratedat 64 in FIG. 6, on a communications line 29, e.g. a dielectric mediumsuch as fiber optics in a specific embodiment, to instruct/condition theswitch control/monitor stage 30 to initiate the transmission of thecombined additional information of the gate signal and the temperatureof the switch stage 10. Thus, the stage 30 sends the normal signals asshown in FIG. 2 until a request signal is received whereupon the signalsdepicted in FIG. 4 are sent, all as depicted in the sequence of FIG. 6.

In accordance with additional aspects of the present invention, thecontroller 18 over the communication lines at 20 is arranged to issuepredetermined ON or OFF signals to control the conductive state of theswitch stages 10 to 10 b over the communications link 29 of thecommunications arrangement 22. In response to the ON or OFF signals at20, the switch control/monitor stage 30 sends a gate drive controlsignal at 24 to turn the switch on or off in accordance with thereceived signal. For example, signals at 20, either on one line or as acoded representation, are responded to by the comm. encoder/mux stage 26which issues an ON signal representation over the communications link 29to the switch control/monitor stage 30. The switch control/monitor stage30 decodes the ON signal representation on the communications link 29and outputs a signal at 24 to the switch stage 10. In one embodiment, amomentary ON signal at 20 causes the stage 30 to turn the switch stage10 on and the switch stage 10 is turned off only upon the issuance of amomentary OFF signal at 20. In another embodiment, the ON signal iscontinuously output at 29 until the switch control/monitor stage 30responds with one or more predetermined signals over the communicationlink 28 to acknowledge that the ON signal has been received and actedupon and/or that the switch stage 10 is conducting, e.g. as shown at 65or 66 in FIG. 5.

In a specific embodiment, the ON/OFF signals at 20 are encoded over thecommunications link 29 as a pulse train of a predetermined number ofpulses, the ON and OFF signals being a different number of pulses. Thecomm. encoder/mux stage 26 encodes the pulse train and the switchcontrol/monitor stage 30 counts the pulses of the signal and determineswhether or not the received signal is an ON or OFF signal. In oneembodiment, the request for diagnostic signal issued by the comm.encoder/mux stage 26 at 29 is a third signal, e.g. a different number ofpulses than the ON or OFF signal representations In another embodiment,the request for diagnostic signal to start the transmission oftemperature signals over the communication link 28 is the transmissionof a predetermined “ON” signal over the link 29. Considering anotherillustrative embodiment of the present invention and referring nowadditionally to FIG. 7, the temperature signal alone is communicated viathe communications arrangement 22 of FIG. 1, e.g. signal 50 for stage10, 54 for stage 10 a, and the signal 60 for ambient temperature at thestage 26. In another embodiment, a distinct ready signal is utilized bythe comm. encoder/mux stage 26 to ready the switch stages 10 x foroperation in response to an ON command being received from thecontroller 18 when the switch stages 10 x are non-conducting. In suchcases, the switch control/monitor stages 30 respond to the detection ofthe distinct ready signal, e.g. predetermined number of pulses at 29, bysending a signal such as 40 in FIG. 1 or 65 or 66 of FIG. 5 over thecommunications link 28. When the signals are received by the comm.encoder/mux stage 26, it can be determined that the switch stages 10 xare ready for operation and ON signals can be issued over thecommunication links 29.

The system 15 in a preferred embodiment is applied to a multi-phaseelectrical power distribution system operating at medium voltages.Accordingly, as shown in FIG. 1, the system 15 includes additional comm.encoder/mux stages 26, e.g. 26-2 and 26-3 for respective second andthird phases of an electrical power source. In one embodiment, thestages 26, 26-2 and 26-3 are connected to receive signals from thecontroller 18 over a common data bus 20 while in other embodiments thesignaling paths are independent. In such systems, when the powerelectronic switch of stages 10, 10 a, 10 b etc. is non-conducting, itmay be desirable to verify its readiness for operation, especially whenit may be called upon for rapid, high-speed operation in a high-speedsource-transfer application. In one embodiment, and referring now toFIG. 8, when the comm. encoder/mux stage 26 receives a signal at 20 fromthe controller 18 representing that the switch stages 10 x are to betested, the comm. encoder/mux stage 26 issues ON commands to a firstportion of the switch control/monitor stages 30, e.g. N/2 where thereare N total switch stages 10 x, or (N+1)/2 where N is an odd number, andthereafter issue ON commands to the remaining switch control/monitorstages 30. Accordingly, the information representing operation of thevarious switch stages 10 x is provided to the controller 18 as shown inFIG. 8, first for the first three stages then for the next three stages.This is useful because a non-conducting switch can be tested while theoverall switch remains non-conducting. Additionally, in a preferredembodiment, the ambient temperature is also provided, as shown at 60 inFIG. 8. As before, in various embodiments, this can be done with thetemperature representations for each stage as shown in FIG. 8 or withoutthe individual temperature representation signals.

Referring now to FIG. 9, a power electronic switching system functioningas a high-speed source transfer switching system (HSSTSS) 110 isillustrative of a specific system application for which the controlarrangement and method of the present invention of FIGS. 1-8 is useful.The HSSTSS 110 supplies a load at 114 with an alternating-currentwaveform via either a first AC source at 116 or a second AC source at118. The first and second AC sources 116 and 118 and the load at 114 asprovided in an electrical power distribution system are typicallymulti-phase circuits which are represented in FIG. 9 by a one-linediagram. The HSSTSS 110 includes a first solid-state switch, SSS1, 120and a second solid-state switch, SSS2, 122, which can also becharacterized as electronic switches or power electronic switches. TheHSSTSS 110 via a system control 112 controls either SSS1 to supply theload at 114 via the first source 116 or controls SSS2 to supply the loadat 114 via the second source 118. In a specific embodiment, the systemcontrol 112 includes the controller 18 of FIG. 1. The system control 112provides appropriate control signals at 128, 130 to control theoperation of each respective solid-state switch, SSS1 120 and SSS2 122.In the specific illustrative embodiment, the system of FIG. 9 utilizesthe communications arrangement 22 of FIG. 1. Accordingly, the controlsignals at 128, 130 are utilized by the communications arrangements 22-1and 22-2 to control the respective solid-state switches SSS1 120 andSSS2 122 over respective gate drive signal arrangements 24-1 and 24-2.

In operation, the system control 112 samples the voltage waveforms ofeach source 116, 118, e.g. via respective sensing inputs at 124, 126 todetect when transfer between the sources is desirable, e.g. sensingoutages and momentary interruptions as well as voltage sags and swellsbased on the source supplying the load being above or below presetlevels. For example, assume that SSS1 120 is turned on by the systemcontrol 112 via signals at 128 so as to be conductive and supply theload at 114. If the system control 112 via the sensing input 124 sensesthat the voltage of the first source at 116 is exhibiting undesirablecharacteristics, the system control 112 via the control signals at 128,130 turns off SSS1 and turns on SSS2 so as to transfer the supply of theload at 114 from the first source at 116 to the second source at 118. Asused herein, the term “incoming” is used to describe the source and theSSS that will be turned on to supply the load (e.g. the second source at118 and SSS2 in the illustrative example), and the term “outgoing” isused to describe the source and the SSS that is being turned off(e.g.the first source at 116 and SSS1 in the illustrative example).

Referring now to FIG. 10, each of the solid-state switches SSS1 and SSS2includes one or more arrays of back-to-back connected thyristors, e.g.140 a and 140 b for SSS1 and 142 a and 142 b for SSS2. In illustrativeimplementations, each array of thyristors is rated in the range of 2-12kv. To provide operation in medium voltage systems, e.g. operating inthe range of 2-34.5 kv, one or more of such thyristors SSS1 and SSS2 areconnected in series for each phase of the sources, e.g. a plurality ofsuch thyristors being referred to as a stack. Thus, while the termthyristor is used for the solid-state switches SSS1, 140 and SSS2, 142,in specific implementations at medium voltages, this commonly refers toa thyristor stack. For example, in a specific embodiment, each of thesolid-state switches SSS1 and SSS2 is implemented by a plurality of theswitch stages 10 x of FIG. 1.

Considering now operation of the control arrangement and method of thepresent invention, transfer of the load at 114 from one source to theother, e.g. the first source at 116 to the second source at 118, isgenerally accomplished by removing the gating signals at 128 a, 128 b toshut off SSS1 and starting the gating signals at 130 a, 130 b to turn onSSS2. Thus, the first source at 116 ceases to supply the load at 114 andthe second source at 118 begins to supply the load at 114. For desirabletransfer control, the controller 112 is provided with additional sensinginputs, e.g. the incoming source-voltage differential is determined bythe load voltage at 114 as sensed via a sensing input 127 or by thedifferential of the source voltages sensed at 124, 126, and the currentto SSS1 and SSS2 being sensed via respective current sensing inputs at129 and 131.

In accordance with additional aspects of the present invention, thesystem control 112 is provided with features to respond to an overheatedcondition of the solid state switches SSS1 and SSS2 to transfer the loadat 114 to the alternate source. For example, if the temperature sensedvia either the communications arrangement 22, or a separate temperaturesense line 150 in a specific embodiment, indicates an overheatedcondition, the system control 112 proceeds with a high-speed transfer.The system control 112 then denotes the alternate source as thepreferred source. The now denoted alternate source with the overheatedswitch is still available on a temporary basis for transfers when thesystem control 112 detects voltage disturbances on the source currentlyfeeding the load such that transfer is required. In an illustrativeembodiment, the overheated condition is defined by any stage of asolid-state switch SSS having a sensed temperature that exceeds theambient temperature by a predetermined differential. i.e. temperaturerise. For example, with reference to FIG. 1, if any electronic switchstage 10 has a sensed temperature at 32 that exceeds the predeterminedlimits, an overheated condition is determined.

When an overheated condition is detected, if it is not possible totransfer to another viable source, the system 110 includes additionalfeatures to initiate and accomplish a backup transfer to bypass andisolate the switches SSS1 and SSS2 of the system 110. Specifically, inan illustrative embodiment, as shown in FIG. 9, to accomplish abypass/isolation sequence, the system controller 112 controls two bypassswitches BP-1 and BP-2 and two isolation switches I-1 and I-2. Theswitches BP-1, BP-2, I-1 and I-2 are controlled via respective controllines 160, 162, 164 and 166. In accordance with additional features ofthe present invention, the bypass/isolation sequence is performed toassure optimum load continuity, e.g. as described by the followingsteps:

Disable high speed transfer control (maintain SSS1, SSS2 states);

Close bypass switch(es) (e.g. BP-1) to match the presently conductingSSS('s), e.g. SSS1;

Confirm that the appropriate bypass switches respond;

Open all isolation switches (e.g. I-1, I-2);

Confirm that the appropriate isolation switches respond;

Remove all gating signals (e.g. at 128, 130) from all SSS's

Enable backup transfer control (e.g. in this case because an SSS isdeemed unusable)

In situations where backup transfer control is enabled, e.g. to performmaintenance or service, an overheated SSS, or otherwise unusable SSS(e.g. due to lack of control), the system control 112 is capable ofproviding source transfer control using the bypass switches BP-1, BP-2,with the isolation switches I-1, I-2 remaining open.

In accordance with additional features of the present invention, whendiagnostic information is received by the system controller 112indicating a potential shorted condition of a switch SSS, e.g. asdetected by the loss of the gating signal 40 or 52 for a particularswitch stage 10 x in FIGS. 1-8, the system controller 112 will identifythe switch SSS and the location of the stage within the switch of thepotential problem. Appropriate flags, alarms etc. are set and issued.However, the system 110 will continue to operate normally and be fullyfunctional since the switches SSS are designed with devices havingsuitable predetermined ratings sufficient to be able to function whenone of the switch stages 10 x is shorted. If diagnostic information isreceived that identifies a potential shorted condition of a second ofthe switch stages 10 x within the same phase or pole of a switch SSS,the system controller 112 initiates the backup transfer mode asdiscussed hereinbefore and the high-speed transfer function is disabled.As discussed hereinbefore in connection with diagnostics of theoperating parameters of the switches such as SSS1 of the system 110 andthe switch stages 10 x of FIG. 1, the loss of the signals 40 or 52indicates that either the switch stage 10 x is shorted, thecommunications arrangement 22 is not functioning or the gate drivesignals at 24 are not functioning.

Considering yet further additional features of the present invention,the system controller 112 also monitors the voltage across each switchSSS that is supposed to be in a conducting mode, i.e. the switch SSSthat is supplying the load at 114. For example, the system controller112 monitors the differential voltage between 116 and 114 for switchSSS1. If the differential voltage is greater than a predetermined value,e.g. 1500v for a 15 kV system, the system controller 112 concludes thatthe there is a malfunction. This detected condition could be caused byan isolation switch being open (which would not be normal), a blown fusein the circuit, or the discontinuity of the switch SSS1 (i.e.non-conducting status such as caused by an open circuit or brokenconnection). If this condition is detected and persists for apredetermined time interval, e.g. 2 milliseconds, the system controller112 initiates a transfer to the second source 118 by turning on theswitch SSS2, and also locks out any transfer back to the switch SSS1. Ofcourse, if for any reason an alternate viable source is not available,the system controller initiates a backup transfer as discussedhereinbefore. In addition or as an alternative to the diagnostic testingof non-conducting switches as discussed hereinbefore, if a switch SSS1has not been turned on in a predetermined period of time, e.g. one day,the system controller 112 initiates a transfer to interrogate the switchSSS1 to verify proper operation to ensure that a viable alternate sourceis available if needed.

While there have been illustrated and described various embodiments ofthe present invention, it will be apparent that various changes andmodifications will occur to those skilled in the art. Accordingly, it isintended in the appended claims to cover all such changes andmodifications that fall within the true spirit and scope of the presentinvention.

What is claimed is:
 1. A method for controlling a source-transferswitching system of the type that includes first and second solid-stateswitches having respective first and second control inputs, input sourceconnections and output connections, the output connections beingconnected to a common load circuit connection, the method comprising:controlling the first and second solid-state switches via the first andsecond control inputs so as to selectively supply the common loadcircuit connection via either the first or second solid-state switch;sensing an overheated condition of each of the first and secondsolid-state switches; responding to the overheated condition of thefirst or second solid state switch supplying the common load circuit byinitiating a transfer to the other of the first or second solid stateswitch; and responding to a subsequent transfer request by initiating atransfer back to the overheated solid-state switch.
 2. A controlarrangement for a source-transfer switching system of the type thatincludes first and second solid-state switches having respective firstand second control inputs, input source connections and outputconnections, the output connections being connected to a common loadcircuit connection, the first and second solid-state switches eachincluding a plurality of series-connected stages, the controlarrangement comprising: transfer control means for controlling the firstand second solid-state switches via the first and second control inputsso as to selectively supply the common load circuit connection viaeither the first or second solid-state switch; first means for sensingone or more improper electrical operating conditions of each of thefirst and second solid-state switches, said first means comprising meansfor sensing said one or more improper operating conditions for eachstage of the first and second solid-state switches, a first of said oneor more improper operating conditions being the shorted condition of anystage of either of the first and second solid-state switches; and secondmeans responsive to a detected improper electrical operating conditionof the first or second solid state switch supplying the common loadcircuit for initiating a transfer to the other of the first or secondsolid state switch, said second means comprising means for determiningwhether or not said sensed improper operating condition is within apredetermined capability of the corresponding first or secondsolid-state switch and initiating said transfer only if not within thepredetermined capability.
 3. The control arrangement of claim 2 whereina second of said one or more improper operating conditions is thenon-conducting status of each stage of the first and second solid-stateswitches.
 4. A control arrangement for a source-transfer switchingsystem of the type that includes first and second solid-state switcheshaving respective first and second control inputs, input sourceconnections and output connections, the output connections beingconnected to a common load circuit connection, the first and secondsolid-state switches each including a plurality of series-connectedstages, the control arrangement comprising: transfer control means forcontrolling the first and second solid-state switches via the first andsecond control inputs so as to selectively supply the common loadcircuit connection via either the first or second solid-state switch;first means for sensing one or more improper electrical operatingconditions of each of the first and second solid-state switches, a firstof said one or more improper operating conditions being thenon-conducting status of any stage of either of the first and secondsolid-state switches, a second of said one or more improper operatingconditions being the shorted condition of any stage of either of thefirst and second solid-state switches; and second means responsive to adetected improper electrical operating condition of the first or secondsolid state switch supplying the common load circuit for initiating atransfer to the other of the first or second solid state switch.